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EPRINTAND  CIRCULAR  SERIES 

OF  THE 

NATIONAL  RESEARCH 
COUNCIL 

SOME  PROBLEMS  OF  SIDEREAL  ASTRONOMY 
By  Henry  Norris  Russell 


NO*/  2  7 

LIBRARY 


Published  in  the  Proceedings  of  the  National  Academy  of  Sci- 
October,  1919,  vol.  5,  no.  10,  pages  391-416 


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REPRINT  AND  CIRCULAR  SERIES 

OF  THE 

NATIONAL  RESEARCH  COUNCIL 

NUMBER  5 

SOME  PROBLEMS  OF  SIDEREAL  ASTRONOMY* 
BY  HENRY  NORRIS  RUSSELL 

Communicated  to  the  National  Academy  of  Sciences,  June  17,  1917 

The  main  object  of  astronomy,  as  of  all  science,  is  not  the  collection  of 
facts,  but  the  development,  on  the  basis  of  collected  facts,  of  satisfactory 
theories  regarding  the  nature,  mutual  relations,  and  probable  history 
and  evolution  of  the  objects  of  study.  Before  the  existing  data  appear 
sufficient  to  justify  the  attempt  to  form  such  a  general  theory,  two 
policies  of  investigation  may  be  followed:  (1)  to  collect  masses  of  in- 
formation, as  accurate  and  extensive  as  possible,  by  well  tested  routine 
methods,  and  leave  it  to  the  insight  of  some  fortunate  and  future  investi- 
gator to  derive  from  the  accumulated  facts  the  information  which  they 
contain  regarding  the  general  problems  of  the  science;  (2)  to  keep  these 
greater  problems  continually  in  mind,  and  to  plan  the  program  of  ob- 
servation in  such  a  way  as  to  secure  as  soon  as  practicable  data  which 
bear  directly  upon  definite  phases  of  these  problems. 

Much  valuable  and  self-sacrificing  work  has  been  done  by  astrono- 
mers who  adopted  the  former  policy.  In  the  opinion  of  many  inves- 
tigators, however,  the  progress  of  astronomy  would  be  hastened  if  fuller 
consideration  were  given  to  the  second  method  of  attack,  especially 
with  a  view  to  the  widest  possible  cooperation  between  different  ob- 
servers and  institutions. 

In  the  hope  that  the  committees  of  the  National  Research  Council 
may  be  of  service  in  furthering  such  cooperation,  and  at  the  request  of 
the  Chairman  of  the  Council,  the  following  survey  has  been  attempted 
of  the  general  problems  of  sidereal  astronomy,  and  of  investigations 
which  at  present  promise  advances  toward  their  solution. 

*  This  is  issued  as  the  first  of  a  series  of  research  surveys  prepared  under  the  auspices  of 
the  National  Research  Council. 

Published  also  in  the  Proceedings  of  the  National  Academy  of  Sciences,  5,  1919.  (391- 
416). 

1 


2  ASTRONOMY:  H.  N.  RUSSELL 

I.  The  Individual  Stars. — Existing  methods  of  investigation  have 
already  put  at  our  disposal  a  great  mass  of  information  regarding  the 
physical  characteristics  of  the  stars — mass,  density,  luminosity,  color, 
spectrum,  temperature,  and  so  on.    The  central  problem  of  stellar 
astronomy  may  be  formulated  as  follows:  From  the  existing  data,  and 
from  all  further  data  which  may  be  secured  by  methods  new  or  old, 
to  deduce  a  theory  of  stellar  evolution,  that  is,  of  the  changes  in  the 
temperature,  density,  brightness,  spectrum,  and  other  observable  char- 
acteristics of  a  star  with  the  progress  of  time,  and  of  the  dependence  of 
these   changes  upon   those   factors  which   are  invariant  for  a  given 
system,  such  as  mass,  angular  momentum  and  chemical  composition. 
Such  a  theory  must  satisfactorily  represent  the  observed  properties  of 
the  general  run  of  the  stars,  and  the  relative  abundance  of  the  different 
types,  and  must  be  capable  of  extension  so  as  to  account  for  the  excep- 
tions to  the  usual  rules. 

Among  the  subsidiary  problems  whose  solution  is  bound  up  with  that 
of  the  main  problem  are  (a)  that  of  the  evolution  of  binary  systems, 
whether  by  fission,  tidal  action,  or  otherwise;  (6)  that  of  the  causes  and 
mechanism  of  variable  brightness  among  the  stars;  (c)  that  of  the  source 
of  the  energy  which  the  stars  radiate  into  space  in  such  enormous 
amounts. 

These  problems  of  stellar  astronomy  are  mainly  physical  in  character, 
though  some  phases,  such  as  (a)  are  mainly  dynamical. 

II.  The  Galactic  System. — The  great  majority,  if  not  all,  of  the  vis- 
ible stars  appear  to  belong  to  an  assemblage  limited  in  space,  either  by 
regions  nearly  void  of  stars,  or  by  absorbing  material  which  conceals 
whatever  may  be  immersed  in  it.     Within  this  galactic  system  we  may 
investigate  the  distribution  of  the  stars  in  space,  and  its  variation  for 
stars  of  different  spectral  type,  absolute  magnitude,  etc.,  the  motions  of 
the  stars  (including  the  Sun),  and  the  phenomena  of  preferential  motion 
('star-streaming')  in  certain  directions,   and  the  dependence  of  these 
motions  upon  spectral  type,  absolute  magnitude,  etc.;  and  the  asso- 
ciation of  the  stars  into  sub-groups,  or  clusters,  and  the  motions  of 
these  clusters. 

All  these  studies  lead  up  to  a  single  ultimate  problem,  which  may  be 
denned  as  the  representation  of  the  present  positions  and  motions  of  the 
stars  as  a  stage  in  the  history  of  a  dynamical  system  (whether  in  a 
steady  state  or  not)  and  the  deduction  of  the  presumable  history  of  the 
system  in  the  past  and  the  future.  Among  the  subsidiary  problems  con- 
nected with  this  are  (a)  the  existence,  character,  distribution  and  gravi- 
tational influence  of  possible  dark  or  absorbing  matter  in  space;  (6)  the' 


ASTRONOMY:  H.  N.  RUSSELL  3 

relation  between  the  age  or  evolutionary  stage  of  a  star  and  its  position 
and  motion  within  the  galactic  systems.  The  latter  connects  the  prob- 
lems of  stellar  and  galactic  evolution  in  such  a  way  that  any  notable 
advance  in  the  solution  of  one  is  likely  to  be  of  aid  in  that  of  the  other, 
while  an  unfounded  assumption  regarding  either  will  probably  confuse 
the  discussion  of  both. 

///.  Clusters  and  Nebulae. — So  little  has  been  known  of  these  objects 
until  very  recently  that  the  problems  which  they  present  can -hardly 
yet  be  coordinated  into  a  single  statement.  Among  the  most  obvious 
are: 

(1)  The  relations  of  dusters  and  nebulae  to  the  galactic  system.    It 
now  appears  probable  that  the  galactic  system  is  very  much  larger  than 
was  supposed  a  few  years  ago,  and  that  not  only  the  irregular  clusters, 
and  the  gaseous  nebulae,  both  planetary  and  extended,  but  also  the 
globular  clusters,  and  probably  the  Magellanic  Clouds,  lie  within  its 
confines.    But  the  relations  of  the  spiral  nebulae  are  still  uncertain. 

(2)  Motions   and   dynamical   relations   within    clusters,    especially 
globular  clusters,  and  explanation  of  the  law  of  distribution  of  stars  in 
such  clusters. 

(3)  Nature  of  the  gaseous  nebulae,  especially  of  'nebulium/  and  cause 
of  their  luminosity.     Internal  motions  in  gaseous  nebulae. 

(4)  Nature  of  spiral  nebulae,  and  explanation  of  the  rapid  motions  of 
their  parts. 

In  all  these  cases  a  persistent  attempt  should  be  made  to  account  for 
the  observed  phenomena  by  means  of  the  known  properties  of  matter 
and  forces  of  nature,  and  the  existence  of  unknown  forces  should  be 
postulated  only  if  there  is  apparently  no  escape  from  the  necessity  of 
doing  so. 

It  may  now  be  profitable  to  survey  rapidly  the  different  fields  of  as- 
tronomical investigation,  and  consider  the  bearing  of  various  researches 
— some  now  under  way,  some  practicable  at  present,  and  others  desir- 
able if  means  for  effecting  them  can  be  devised — upon  these  general 
problems. 

L  Spectra. — It  seems  to  be  increasingly  dear  that  the  master-key 
to  these  problems,  so  far  as  they  have  yet  been  formulated,  lies  in  the 
investigation  of  the  spectra  of  the  stars  and  other  bodies,  and  the  cor- 
relation of  their  other  characteristics  with  the  spectra.  Fortunately, 
the  spectra  are  among  the  few  characteristics  which  can  be  investigated 
independently  of  any  knowledge  of  the  distances  of  the  various  bodies, — 
and,  indeed,  of  the  distances  themselves,  except  for  the  limitation  arising 
from  the  faintness  of  most  of  the  remoter  objects. 


4  ASTRONOMY:  H.  N.  RUSSELL 

(a)  Two  fundamental  facts  appear  upon  the  study  of  the  lines  of  stellar 
spectra.    The  first  is  that  almost  all  of  the  thousands  of  lines  which 
have  been  observed  are  identifiable  as  those  of  known  elements,  and 
can  be  reproduced  under  conditions  which  can  be  realized  in  terrestrial 
laboratories.     The  few  outstanding  exceptions  are  yielding  year  by  year. 
The  recent  identification  of  the  G  band  in  the  solar  spectrum  as  due  to 
hydrocarbons,1  and  of  the  bands  of  ammonia2  and  water-vapor3  in  the 
ultra-violet,  leaves  very  few  'unknown'  solar  lines  of  any  importance. 
Nor  are  there  any  of  great  account  in  stellar  spectra,  except  in  stars  of 
the  fourth  type  (Class  N)  and  in  the  Wolf-Rayet  and  'early'  helium 
stars. 

So  many  of  the  lines  in  the  latter  have  recently  been  found  to  be 
identical  with  those  given  in  the  laboratory  by  familiar  elements  (such 
as  hydrogen,  oxygen,  carbon,  and  helium),  under  unusually  intense 
electrical  excitation4  that  there  is  good  reason  to  hope  that  further  re- 
searches in  this  direction  may  account  for  those  which  still  remain, 
and  even  solve  the  long-standing  riddle  of  the  origin  of  the  characteris- 
tic nebular  lines  (which  are  associated  with  the  Wolf-Rayet  lines  in 
the  nuclei  of  planetary  nebulae  and  in  new  stars  at  certain  stages). 
The  spectrum  of  the  solar  corona,  however,  still  remains  an  isolated 
problem. 

(b)  The  second  great  fact  is  that  the  vast  majority  of  stellar  spectra 
fall  into  a  single,  continuous,  linear  sequence,  which  forms  the  basis  of 
the  Harvard  system  of  classification,  now  generally  adopted.    Almost 
all  the  spectra  which  did  not  obviously  belong  to  this  sequence  have  been 
brought  into  connection  with  it  by  the  recent  work  of  Wright,5  con- 
necting the  gaseous  nebulae  with  the  Wolf-Rayet  stars  at  the  head  of 
the  series,  and  that  of  Curtiss  and  Rufus,8  which  shows  that  the  small 
but  definite  classes  R  and  N  form  a  sort  of  side-chain,  branching  from 
the  main  sequence  near  the  other  end,  at  class  G  (or  perhaps  K).     Miss 
Cannon's  experience7  in  classifying  over  200,000  spectra  shows  that  the 
objects  that  do  not  fall  into  the  sequence,  thus  extended,  are  almost 
vanishingly  rare. 

The  general  characteristics  of  this  sequence  are  now  well  established, 
and  the  types  which  were  selected,  by  a  sort  of  survival  of  the  fittest, 
in  the  evolution  of  the  Harvard  classification  prove  to  have  been  sur- 
prisingly well  distributed  along  the  series.  With  the  aid  of  the  quan- 
titative methods  of  classification  developed  by  Adams  and  Kohl- 
schiitter,8  the  precise  classification  of  any  spectrum  of  which  a  good 
photograph  with  suitable  dispersion  is  available  should  be  an  easy 


ASTRONOMY:  H.  N.  RUSSELL  5 

matter,  even  in  the  interval  between  G  and  K5,  where  the  differences 
between  consecutive  types  are  least  prominent.  The  publication  of  a 
detailed  descriptive  'key'  with  good  reproductions  of  photographs  of 
spectra  of  each  successive  class  would  however  be  a  great  boon  to 
isolated  workers. 

Of  much  greater  importance  is  the  devising  of  some  method  for  pho- 
tographing the  spectra  of  stars  fainter  than  the  tenth  magnitude — 
which  are  now  about  at  the  limit  of  accessibility.  Long  exposures 
with  the  objective  prism  are  greatly  embarrassed  by  difficulties  in 
guiding,  but  the  problem  is  doubtless  soluble  in  some  way,  and  ought 
to  be  solved. 

(c)  There  is  now  good  reason  to  believe  that  the  differences  between 
the  main  classes  of  spectra  arise  from  differences  in  the  effective  surface 
temperatures  of  the  stars,  and  that  differences  in  their  other  physical 
characteristics  play  only  a  minor  rdle  in  the  spectra,  but  reveal  them- 
selves in  differences  in  detail,  formerly  described  as  'peculiarities' 
when  they  were  noticed  at  all.  The  investigation  of  these  finer  differ- 
ences is  to-day  the  most  promising  field  in  stellar  spectroscopy. 

What  valuable  results  may  be  obtained  was  shown  by  Hertzsprung's9 
work  on  Miss  Maury  V  c-stars'  (with  unusually  sharp  spectral  lines) 
which  prove  to  be  of  greater  real  brightness  than  any  other  class  of 
stars  so  far  known;  and  later,  and  still  more  remarkably,  by  Adams' 
and  Kohlschiitter's10  discovery  that  the  absolute  magnitudes  of  stars 
(of  the  'later'  spectral  classes,  at  least)  can  be  predicted  with  surprising 
accuracy  from  the  relative  intensity  of  a  few  pairs  of  lines  in  their  spectra. 
The  data  for  stars  of  great  luminosity  are  still  scanty,  but  should  be 
easily  obtainable,  using  the  hundreds  of  spectrograms  now  available  at 
the  great  observatories,  and  determining  the  mean  absolute  magnitude 
of  groups  of  stars,  which  the  spectroscopic  method  indicates  as  being 
of  similar  brightness,  by  means  of  their  parallactic  motions.  When 
this  has  been  done,  our  knowledge  of  the  distribution  of  the  naked-eye 
stars  in  space  will  be  very  greatly  advanced. 

The  careful  comparison  of  the  spectra  of  pairs  of  stars  otherwise  simi- 
lar, but  known  to  differ  in  other  characteristics  than  absolute  magnitude, 
may  yield  results  of  importance.  Many  recognizable  spectral  'peculi- 
arities' too,  such  as  the  diffuseness  or  sharpness  of  the  lines,  the  pres- 
ence of  bright  lines,  the  abnormal  strength  or  weakness  of  certain 
lines,  etc.,  have  as  yet  been  very  incompletely  studied,  especially  as 
regards  their  relation  to  other  characteristics  of  the  stars.  For  ex- 
ample, it  should  be  possible  to  distinguish  between  widening  of  spectral 
lines  due  to  a  star's  rotation,  (which  would  affect  all  lines  alike),  and 


6  ASTRONOMY:  H.  N.  RUSSELL 

widening  due  to  physical  conditions  in  its  atmosphere  (which  are  likely 
to  affect  some  lines  more  than  others). 

(d)  Another  promising  field  is  found  among  the  reddest  stars.  Curtiss 
makes  the  very  interesting  suggestion  that  the  division  of  the  spectral 
series  into  the  branches  G-K-M  and  G-R-N  (or  perhaps  K-R-N) 
may  be  due  to  differences  of  chemical  composition11 — since  it  is  known 
that  the  surface  temperatures  of  these  stars  are  low  enough  to  permit 
the  formation  of  chemical  compounds.  If  this  is  true,  the  strength 
of  the  characteristic  bands  of  titanium  oxide  or  of  carbon  should  depend 
upon  the  relative  proportions  of  these  elements,  and  show  little  correla- 
tion with  the  color  index,  or  the  extension  of  the  spectrum  in  the  violet, 
which  depend  primarily  on  the  temperatures.  There  is  already  consid- 
erable evidence  that  this  is  actually  the  case,  and  it  may  be  remarked 
that  the  star  Epsilon  G'eminorum,  which  is  of  spectral  class  G5  has  a 
color  index  (  +  1.52)  almost  equal  to  that  of  Classes  M  or  R.12  This 
star  may  be  in  the  situation  anticipated  by  Curtiss,  in  which  an  exact 
chemical  equilibrium  between  carbon  and  titanium  oxide  suppresses 
the  bands  of  both. 

Photography  of  the  spectra  of  bright  stars  in  the  red,  and  even  the 
near  infra-red,  is  now  practicable,  and  Merrill13  has  already  obtained 
results  of  great  interest  and  promise.  Investigation  of  the  spectra  of 
the  brightest  stars  with  high  dispersion  is  also  profitable,  as  is  shown 
by  the  work  of  Adams14  upon  the  pressures  which  probably  prevail  in 
the  atmospheres  of  Sirius,  Procyon,  and  Arcturus.  Fortunately,  the 
stars  brighter  than  the  second  magnitude  afford  examples  both  of  giant 
and  dwarf  stars  of  almost  every  spectral  class. 

2.  (a)  Almost  equal  in  importance  to  the  line  absorption  in  stellar 
spectra  is  the  distribution  of  intensity  in  the  continuous  background.  The 
most  complete  and  satisfactory  method  of  studying  this  would  be  the 
direct  measurement  of  the  energy  carried  by  different  wave-lengths,  but 
this  has  not  yet  been  proved  practicable.  A  first  step  has  however  been 
taken  by  Coblentz,15  who  has  not  only  measured  the  total  energy  radia- 
tion of  more  than  a  hundred  stars,  but  in  some  cases  the  percentage 
transmitted  by  a  water  cell,  thus  providing  our  first  knowledge  of 
stellar  radiation  in  the  infra-red.  With  the  great  reflectors  just  com- 
pleted, the  determination  of  spectral  energy  curves  for  the  brightest 
stars  may  be  possible. 

The  distribution  of  energy  in  the  luminous  region  of  the  spectrum  is 
however  readily  determinable.  For  the  brighter  stars,  spectro-photo- 
metric  methods  can  be  employed,  as  in  the  visual  work  of  Wilsing 


ASTRONOMY:  H.  N.  RUSSELL  7 

and  Scheiner,16  and  the  photographic  investigations  of  Rosenberg.17 
Fainter  stars,  down  to  the  sixteenth  magnitude,  at  least,  can  be 
reached  by  the  determination  of  color  indices. 

(b)  In  order  that  these  color  indices  may  be  capable  of  full  utiliza- 
tion, it  is  necessary,  first,  that  trustworthy  and  homogeneous  scales  of 
visual,  photographic  and  photovisual  magnitudes  be  established  over 
the  whole  range  of  about  47  magnitudes  from  the  Sun  to  the  faintest 
observable  stars.  This  problem,  which  is  fundamental  in  all  stellar 
photometry,  is  already  well  advanced  toward  solution.  But  in  the 
second  place,  it  is  necessary  that  the  physical  meaning  of  the  units  of 
magnitude  should  be  precisely  known;  that  is,  that  the  'luminosity 
curve'  which  expresses  the  relative  sensitiveness  of  the  photometric 
receiver  for  equal  energy  of  different  wave-lengths  should  be  exactly 
determined.  And,  above  all,  it  is  essential  that  this  luminosity  curve 
should  be  independent  of  the  brightness  of  the  stars  under  observation. 
These  last  two  conditions  are  at  present  very  imperfectly  satisfied,  if 
at  all.  Very  little  is  known  about  the  luminosity  curves  of  the  standard 
plates  and  apparatus  used  in  the  determination  of  photographic  and 
photovisual  magnitudes,  and  nothing  at  all  about  the  luminosity  curves 
of  the  eyes  of  the  'standard  observers'  at  different  observatories,— 
except  that  they  must  be  very  different  under  the  conditions  prevailing 
at  Harvard  and  at  Potsdam.18  It  is  certain  that  the  Purkinje  effect 
alters  the  form  of  the  visual  luminosity  curve  as  the  brightness  of  the 
illumination  varies,  probable  that  this  affects  the  visual  comparison  of 
the  brightness  of  stars  of  widely  different  magnitudes,  and  uncertain 
whether,  and  to  how  great  an  extent,  similar  photographic  influences 
exist.19 

The  direct  determination  of  the  luminosity  curves  for  the  principal 
instruments  and  methods  employed  in  the  determination  of  photo- 
graphic and  photovisual  magnitudes  would  be  neither  difficult  nor  la- 
borious. For  visual  observations  it  can  be  der  ved  indirectly,  if  direct 
measures  prove  difficult.  To  make  these  investigations  at  once  is 
urgently  desirable,  for  the  present  bases  oi  the  scales  of  stellar  magni- 
tude are  not  permanent.  The  photographic  and  photovisual  scales 
depend  on  the  properties  of  present  commercial  types  of  rapid  plates, 
which  may  not  be  manufactured  a  few  years  hence  if  improvements  are 
devised;  and  the  visual  scales  are  based  on  the  characteristics  of  the  eyes 
of  observers  some  of  whom  have  already  retired  from  active  work. 

Such  an  investigation  would  also  establish  a  connection  between  the 
scales  of  stellar  magnitude  and  the  physical  units  of  measurement  of 
light  in  the  laboratory  (which  are  now  defined  in  terms  of  a  definite 


8  ASTRONOMY:  H.  N.  RUSSELL 

luminosity  curve),  and  would  enable  us  to  express  our  stellar  photo- 
metric data  in  absolute  units. 

It  is  also  desirable  that  methods  for  measuring  the  brightness  of  the 
stars  with  red  and  ultra-violet  light  should  be  developed,  with  careful 
determination  of  the  luminosity  curve  in  each  case,  and  of  the  color 
equation  which  (for  normal  stars)  makes  it  possible  to  reduce  color- 
indices  obtained  on  any  of  these  systems  to  a  standard  scale. 

The  determination  of  the  colors  of  faint  stars  by  other  methods 
affords  a  promising  field,  as  is  shown  by  the  success  of  the  method  of 
effective  wave-lengths,20  and  of  that  of  exposure  ratios,21  recently 
developed  at  Mount  Wilson. 

Such  a  determination  of  exact  scales  of  magnitude  and  color  index  is 
evidently  a  necessary  condition  for  the  full  utilization  of  the  great  mass 
of  material  which  is  in  process  of  collection  concerning  the  numbers  of 
stars  of  different  magnitudes,  their  concentration  towards  the  Galaxy,  etc. 

(c)  The  statistical  investigation  of  the  relations  between  color  index 
and  spectral  type,  and  between  both  and  absolute  magnitude,  have 
already  opened  up  possibilities  of  estimating  the  distances  of  stars  far 
too  remote  to  be  reached  in  any  other  way.     Such  investigations 
should  be  extended,  with  special  reference  to  stars  of  great  and  small 
absolute  brightness,  and  to  those  having  peculiar  spectra. 

Closely  connected  with  this  is  the  question  of  possible  selective  ab- 
sorption of  light  in  space.  Shapley's  results,22  and  the  theoretical 
work  of  L.  V.  King,23  appear  to  negative  the  existence  of  any  general 
absorption  of  this  sort.  But  local  selective  absorption  may  occur,  and 
it  would  be  well  worth  while  to  study  intensively  the  color  indices  and 
spectra  of  stars  in  regions  where  the  existence  of  absorbing  matter  is 
suspected,  such  as  Barnard's  dark  lanes  in  Scorpius.  It  is  interesting 
in  this  connection  to  note  that  the  three  most  abnormally  yellow  stars 
of  Class  B  (f ,  o  and  £  Persei)24  lie  within  5°  of  one  another,  in  a  region 
full  of  diffuse  nebulosity.25  A  survey  of  the  stars  in  this  region  for 
color-index  and  spectral  type  would  be  well  worth  while. 

(d)  Another  interesting  problem  is  presented  by  the  extreme  infre- 
quency  of  very  red  stars.    Color-indices  up  to  about  + 1 .8  on  the  Harvard 
scale  are  fairly  common;  but  greater  values  are  very  unusual,   and  are 
practically  confined  to  the  'side  chain'  which  includes  Class  N.    In 
this  subsidiary  sequence  the  color-indices  increase  to  about  +4,  as 
might  be  expected  as  a  result  of  decreasing  temperature;  but  in  the 
main  series,  ending  in  Class  M,  this  does  not  happen.     Are  all  the 
stars  of  Class  M  of  about  the  same  temperature,  or  is  an  increase  of 
redness  in  Classes  Mb  and  Me  masked  by  increasing  absorption  in  the 


ASTRONOMY:  H.  N.  RUSSELL  9 

red  end  of  the  spectrum?  There  are  certainly  very  heavy  absorption 
bands  in  the  red  in  these  spectra;  and  further  evidence  in  favor  of  this 
hypothesis  is  found  in  Coblentz's  measures  of  Alpha  Herculis,26  which 
show  this  star,  of  Class  Me,  radiates  far  more  heat  in  proportion  to  its 
light  than  do  stars  of  Class  Ma,  and  also  in  Hertzsprung's27  observation 
that  the  very  faint  dwarf  stars  of  Class  Mb  are  not  nearly  as  red  as  their 
small  luminosity,  and  probable  low  surface  brightness,  would  lead  one  to 
suppose.  A  careful  study  of  the  color  indices,  and,  if  possible,  of  the 
spectral  energy  curves,  of  the  stars  of  Classes  Ma,  Mb,  and  Me  is  much 
to  be  desired.  The  extraordinarily  red  stars  S  Cephei28  and  +43°53,29 
which  have  color  indices  exceeding  five  magnitudes,  should  be  included 
in  such  a  study. 

3.  One  other  stellar  characteristic  which  may  be  investigated  without 
knowledge  of  distance  is  variability  of  brightness.  If  we  really  under- 
stood the  causes  of  stellar  variability,  we  should  probably  have  ad- 
vanced a  long  way  towards  the  solution  of  the  whole  problem  of  stellar 
evolution,  if  not  have  solved  it  completely.  But,  in  spite  of  the  great 
number  of  variable  stars,  the  variety  of  the  phenomena  which  they 
represent,  and  the  accuracy  with  which  they  can  now  be  observed,  the 
humiliating  admission  must  be  made  that  no  even  tolerably  satisfactory 
theory  of  the  causes  of  the  variation  exists,  except  for  the  eclipsing 
Variables,  and  in  this  case  it  is  based  on  the  proposition  that,  except 
for  the  accident  of  eclipse,  the  components  are  not  variable  at  all! 

Successful  attack  upon  the  problem  of  intrinsic  stellar  variation  will 
probably  demand  the  correlation  of  all  the  data  that  can  be  brought 
together  from  every  accessible  source.  In  the  case  of  regular  variables, 
precise  light  curves  are  of  importance,  and  many  stars  still  await  in- 
vestigation,— some  of  them  visible  to  the  naked  eye,  and  long  known  to 
be  variable.  The  new  photometric  methods  of  precision — especially 
the  photoelectric  cell — have  opened  a  wide  field  in  the  study  of  bright 
stars  with  small  variation,  in  which  important  results  have  already 
been  obtained, — notably  by  Stebbins30  and  Guthnick,31 — and  more  may 
be  anticipated. 

(a)  Former  suspicions  of  changes  in  form  of  the  light  curves  appear 
to  have  been  unfounded  in  the  case  of  eclipsing  variables;  but  similar 
changes  are  believed  with  good  reason  to  exist  among  Cepheid  vari- 
ables.32   To  prove  their   reality — still  more  to  discover  their  laws — 
demands  very  precise  observations,  preferably  by  two  or  more  observers 
at  different  places  and  the  same  time. 

(b)  Changes  in  color,  as  well  as  in  brightness,  appear  to  be  the  general 
— perhaps  the  invariable,  rule  among  eclipsing  variables,  and  especially 


10  ASTRONOMY:  H.  N.  RUSSELL 

among  Cepheids — the  star  being  always  redder  at  minimum  than  at 
maximum.  More  recent  observations  show  that  changes  in  the  spectrum 
go  hand  in  hand  with  the  others. 

In  the  case  of  eclipsing  variables,  these  changes  arise  from  a  difference 
in  spectral  type  between  the  components,  and  it  is  found  that  stars 
separated  by  an  interval  less  than  their  own  diameters,  and  therefore 
very  probably  of  the  same  origin  and  age,  may  have  spectra  differing 
as  widely  as  those  of  Sirius  and  Arcturus.33  Observations  of  such  sys- 
tems, when  the  eclipse  is  total,  provide  the  only  direct  method  at 
present  existing  for  studying  the  relations  between  spectral  type,  color 
index,  surface  brightness,  and  density,  which  are  of  fundamental  im- 
portance. The  determination  of  the  spectral  type  of  the  fainter  com- 
ponents of  such  systems,  though  often  very  difficult,  on  account  of  their 
extreme  faintness,  deserves  special  effort. 

(c)  The  concomitant  variations  in  brightness,  color,  and  spectrum, 
which  Shapley34  has  shown  to  occur  in  every  Cepheid  variable  that  has 
been  properly  investigated,  indicate  very  strongly  that  the  proximate 
cause  of  the  changes  in  all  three  is  a  periodic  variation  in  the  surface 
temperature  of  the  stars.     Shapley's  suggestion35  that  these  differences 
in  temperature  arise  from  some  sort  of  internal  changes,  perhaps  of  the 
nature  of  periodic  oscillations  in  the  radius,  density,  temperature,  etc., 
appears  to  be  the  best  which  has  been  yet  made;  but  there  are  still  grave 
difficulties  in  explaining  how  such  pulsations  should  in  all  cases  pro- 
duce the  very  distinctive  form  of  the  light  curve,  with  itp  rapid  rise  and 
slow  fall,  and  still  greater  trouble  in  accounting  for  the  variations  in 
radial  velocity,  which  show  so  remarkable  a  relation,  both  in  amplitude 
and  phase,  to  those  in  light.    It  is  in  fact  still  doubtful  whether  these 
stars  are  really  binary  systems  or  not.    Intensive  studies  of  a  number 
of  these  variables,  including  the  greatest  practicable  variety  of  represen- 
tative cases,  would  be  well  worth  while. 

(d)  Still  less  is  known  concerning  the  very  numerous  variables  of  long 
period,  and  the  roughly  periodic  and  irregular  variables.    In  the  obser- 
vation of  their  changes  in  brightness,  amateur  observers  may  obtain 
results  of  much  value,  and,  under  the  admirable  cooperative  schemes 
organized  by  the  American  and  British  Associations  of  Variable  Star 
Observers,  they  are  at  present  furnishing  a  great  mass  of  valuable  in- 
formation.   Very  little  is  known  regarding  changes  in  the  spectra  of 
long-period  variables,  except  that  they  often  exist,86  especially  as  re- 
gards the  bright  hydrogen  lines  which  are  usually  present  at  maximum. 
Observations  of  the  color  indices  of  these  variables  are  also  much  to  be 
desired.     Certain  peculiar  variables,  such  as  R  Coronae  and  SS  Cygni, 


ASTRONOMY:  H.  N.  RUSSELL  11 

are  typical  of  small  but  definite  groups,  whose  variation,  though  quite 
distinctive,  is  entirely  unpredictable.  The  spectra  of  the  stars  of  the 
first  of  these  groups  are'  similar  to  one  another,  and  unlike  anything 
else.36  Those  of  the  second  group  are  also  peculiar,  and  appear  to  be 
variable.36  Both  present  problems  as  alluring  as  they  are  difficult. 
The  spectra  of  other  peculiar  variables  also  deserve  investigation. 

(e)  New  stars  are  usually  pretty  fully  observed  while  they  remain 
bright,  but  work  remains  to  be  done  in  following  at  shorter  intervals 
the  changes  during  their  later  stages.  The  recent  work  of  Adams  and 
Pease37  indicates  that  they  settle  down  into  Wolf-Rayet  stars;  but, 
according  to  Miss  Cannon,38  the  spectrum  of  the  Nova  in  Corona,  fifty 
years  after  its  outburst,  is  now  of  class  K.  No  one  seems  yet  to 
have  followed  up  Hertzsprung's  interesting  suggestion39  that  stars  of 
very  small  absolute  luminosity  should  be  investigated  for  variability. 
Abundant  material  for  a  photographic  study  must  exist  in  the  Harvard 
collection. 

4.  Knowledge  of  the  distances  of  the  stars  is  indispensable  in  the 
solution  of  many  problems.  The  nearer  ones,  to  a  distance  of  thirty 
parsecs  or  so,  are  now  accessible  to  direct  measures  of  parallax,  and 
great  activity  prevails  in  photographic  observation  for  this  purpose,  in 
accordance  with  a  wide  and  well-considered  plan  of  cooperation. 

In  my  opinion,  however,  the  greatest  need  in  parallax  work  at 
present  is  the  investigation  and  elimination  of  the  systematic  errors 
which  are  still  present  in  the  best  work,  as  is  shown  by  the  too 
frequent  appearance  of  large  discordances — sometimes  amounting  to 
more  than  0''05 — between  the  results  of  different  observers,  although 
the  probable  errors  derived  from  the  internal  agreement  of  each  ob- 
server's plates  are  of  the  order  of  ±0''01.  The  intercomparison  of  the 
results  of  various  observers  for  the  same  stars  is  hardly  a  sufficient 
test  for  the  absence  of  systematic  error,  especially  as  all  are  using  nearly 
the  same  method  of  observation.  The  only  secure  control  is  afforded 
by  observing  stars  whose  parallaxes  can  be  predicted,  from  other  con- 
siderations, with  greater  accuracy  than  they  can  be  observed.  This 
demands  prediction  with  a  probable  error  not  exceeding  ±0f005. 
Fortunately,  several  groups  of  stars  exist  for  which  such  prediction  is 
possible.  The  most  prominent  of  these  consists  of  those  stars  of  spec- 
trum B  which  are  between  60°  and  120°  from  the  solar  apex.  If  the 
parallaxes  of  these  stars  are  computed  on  the  assumption  that  their 
individual  proper  motions  are  entirely  due  to  the  solar  motion,  the 
resulting  errors  will  correspond  to  a  probable  error  of  less  than  one- 
third  of  the  parallaxes  themselves — that  is,  to  about  ±0'!002.  The 


12  ASTRONOMY:  H.  N.  RUSSELL 

stars  of  Kapteyn's  Scorpius-Centaurus  group40  would  be  ideal  objects, 
if  they  were  not  too  far  south. 

For  fainter  stars,  eclipsing  and  Cepheid  variables  are  available.  Of 
the  90  eclipsing  variables  whose  parallaxes  were  estimated  by  Russell 
and  Shapley,41  69  are  fainter  than  the  eighth  magnitude,  and  the 
mean  parallax  of  these  is  0*002,  while  only  ten  per  cent  exceed  OT004. 
For  the  Cepheids  of  similar  brightness,  the  parallaxes  estimated  by 
Hertzsprung  and  Shapley42  are  even  smaller. 

When  once  the  systematic  errors  have  been  tracked  to  their  source 
and  eliminated,  an  extensive  program  of  observation  can  be  undertaken 
with  security.  Much  duplication  of  observations  is  desirable,  for  it  is 
obviously  better  that  the  parallax  of  a  star  should  be  determined  from 
the  mean  of  three  or  four  short  series  of  as  many  different  observatories 
than  by  a  series  with  a  single  instrument,  however  long  and  elaborate. 
Certain  objects  for  which  especially  accurate  parallaxes  are  desirable 
should  be  observed  at  as  many  places  as  possible.  Examples  are  binary 
stars,  stars  differing  in  absolute  magnitude  from  the  bulk  of  those  of 
the  same  spectral  class,  or  from  the  values  predicted  by  the  spectro- 
scopic  method,  stars  with  exceptionally  rapid  motions  in  space,  planetary 
nebulae,  etc.  Attempts  to  determine  by  direct  observation  the  mean 
difference  in  parallax  between  classes  of  stars  with  small  parallaxes  (for 
example,  those  of  the  third  and  fourth  magnitudes,  taken  as  a  whole) 
should,  in  my  judgment,  be  deferred  until  the  systematic  errors  have 
been  thoroughly  cleaned  out. 

5.  Knowledge  of  parallax  leads  at  once  to  that  of  absolute  magnitude, 
which,  in  the  interest  and  importance  of  its  systematic  relations  to 
other  characteristics  of  the  stars,  stands  second  only  to  spectral  type. 

(a)  The  relations  between  the  two  afford  a  very  interesting  study, 
which  has  led  Hertzsprung43  and  Russell44  to  the  recognition  of  the  two 
series  of  'giant'  and  'dwarf  stars,  coincident  in  class  B,  but  gradually 
drawing  apart  among  the  redder  stars  until,  as  Adams'  spectroscopic 
results  have  recently  confirmed,45  they  are  completely  and  widely  sepa- 
rated in  class  M.  If  Russell's  views  are  correct,  the  existence  of 
these  two  series  is  the  key  to  the  problem  of  stellar  evolution.  In  any 
case,  their  existence  must  be  accounted  for,  and  will  be  of  importance 
in  testing  any  theory.  The  securing  of  additional  data,  especially 
regarding  the  absolute  magnitudes  of  individual  giant  stars,  is  much 
to  be  desired.  It  is  of  importance  to  determine  not  only  the  mean 
absolute  magnitude  of  the  giant  and  dwarf  stars  of  each  spectral  class 
(whenever  the  two  are  separated)  but  the  dispersion  of  the  individual 
values  about  the  mean.  Only  when  the  latter  is  known  can  the  results 


ASTRONOMY:  H.  N.  RUSSELL  13 

of  statistical  investigations  be  cleared  from  the  effects  of  the  egregious 
observational  preference  for  the  brighter  and  remoter  stars. 

(b)  Kapteyn48  has  obtained  fairly  good  values  of  the  dispersion  among 
the  various  divisions  of  Class  B,  and  provisional  values  for  Class  A; 
and  Russell47  has  given  rough  estimates  for  the  dwarf  stars,  and  a  still 
rougher  one  for  the  giants  of  Class  M :  but  further  work  is  greatly  needed. 
Adams'  spectroscopic  method  offers  an  easy  solution  of  the  problem,  as 
soon  as  his  present  provisional  scale  of  absolute  magnitudes  for  the  giant 
stars  has  been  revised  with  the  aid  of  studies  of  the  parallactic  and 
peculiar  motions  of  groups  of  stars  whose  spectra  indicate  that  they  are 
similar  in  real  brightness.     Stromberg48  has  already  shown  in  this  way 
that  Adams'  mean  absolute  magnitude  for  all  the  giant  stars,  taken 
together,  is  substantially  correct;  but  there  is  evidence  that  the  pro- 
visional estimates  for  the  very  brightest  stars  (such  as  the  Cepheid 
variables)  make  them  considerably  too  faint.49 

(c)  The  existing  evidence  indicates  that  the  majority  of  the  stars  of 
any  given  spectral  class  are  confined  within  surprisingly  narrow  limits  of 
absolute  magnitude  (provided  that  the  giants  and  dwarfs  can  be  treated 
separately).    But  there  are  exceptions  of  great  interest.    For  example, 
Kapteyn50  has  shown  that  /3  Orionis  is  some  eight  magnitudes  brighter 
than  the  average  for  its  class  (B8);  and  the  faint  companions  of  Sirius51 
and  o2  Eridani61  have  spectra  of  class  A,  although  they  are  at  least  eight 
magnitudes  iainter  than  normal  stars  of  this  class.    Exceptional  bright- 
ness is  probably  explicable  by  unusual  size  or  mass;  but  the  two  excep- 
tionally faint  stars  (which  are  known  to  be  of  normal  mass  for  stars  of 
their  brightness)  present  a  real  puzzle.     Something  about  the  physical 
conditions  in  these  stars  must  be  very  unusual,  and  they  should  be 
studied  with  the  greatest  attainable  detail.    Other  such  objects  may  be 
found  among  the  faint  stars  of  large  proper  motion.98 

6.  Beyond  the  limit  of  direct  measures  of  parallax,  our  main  reliance 
must  be  placed  on  proper  motions,  which  are  of  fundamental  importance 
in  the  study  of  the  galactic  system. 

The  brighter  stars  have  already  been  cared  for  by  Boss,  and  those 
down  to  magnitude  7.5  are  under  discussion.  The  fainter  stars  can  best 
be  investigated  by  photography,  carrying  the  work  to  objects  as  faint 
as  can  be  reached  with  large  instruments,  in  accordance  with  Kapteyn's 
'Plan  of  Selected  Areas'  or  some  equivalent.  For  this  purpose,  it  is 
essential  to  have  a  set  of  reference  stars,  distributed  uniformly  over  the 
sky,  and  of  suitable  brightness  to  serve  as  photographic  standards,  and 
to  make  the  observations  strictly  differential  with  respect  to  these, 
using  them  not  merely  as  reference  points  for  position  when  reducing  a 


14  ASTRONOMY:  H.  N.  RUSSELL 

single  plate,  but  as  reference  points  for  proper  motion  when  comparing 
two  plates  of  different  epochs.  The  observations  of  these  reference  stars 
must  at  present  be  made  with  meridian  circles;  but  the  proposed  methods 
for  determination  of  absolute  positions  of  the  stars  by  photography 
deserve  careful  study  and  trial. 

Pending  the  completion  of  such  a  program,  the  investigation  of  the 
proper  motions  of  faint  'optical'  companions  of  bright  stars,  such  as 
has  been  made  by  Comstock,53  furnishes  our  best  source  of  information 
concerning  the  proper  motions  of  faint  stars,  but  is  complicated  by 
systematic  errors  in  the  early  measures.  A  survey  of  the  whole  heavens 
for  stars  of  large  proper  motion  is  very  desirable.  In  this  case  it  is 
legitimate  to  treat  the  general  'background'  of  stars  as  at  rest,  and  the 
observations  can  be  very  rapidly  made,  with  the  blink  microscope  or 
similar  appliances.  Early  plates  are  probably  already  available  for 
almost,  if  not  quite,  the  whole  of  the  heavens.  Such  an  investigation 
is  likely  to  yield  important  information  concerning  the  stars  of  very 
small  absolute  luminosity — as  is  shown  by  Barnard's54  and  Innes's68 
recent  remarkable  discoveries — and  should  be  extended  to  the  faintest 
accessible  stars. 

Comparison  of  measures  of  plates  taken  at  different  epochs  (still 
treating  the  bulk  of  the  stars  as  fixed)  will  yield  much  information  about 
proper  motions  of  moderate  size.  This  has  already  been  done  on  an 
extensive  scale  with  plates  of  the  Astrographic  Catalogue. 

Special  investigations  should  be  made  to  determine  at  an  early  date  the 
proper  motions  of  all  stars  belonging  to  certain  interesting  classes  for 
which  early  determinations  of  position  are  available — for  example, 
binaries,  variables,  and  stars  having  peculiar  spectra. 

7.  The  study  of  the  radial  velocities  of  the  stars  is  intimately  associated 
with  that  of  the  proper  motions.  The  determination  of  radial  velocities 
with  the  slit  spectroscope  has  been  brought  to  a  high  degree  of  perfec- 
tion, but  the  separate  investigation  of  each  one  of  the  many  thousands 
of  stars  which  are  now  accessible  would  involve  an  enormous  amount 
of  labor.  The  development  of  some  method  by  which  radial  velocities 
could  be  determined  en  masse  with  the  objective  prism  would  be  a 
great  boon.  If  some  absorbing  medium  giving  sharp  and  well  dis- 
tributed lines  in  the  blue  and  violet  could  be  found,  the  problem  would 
become  simple;  and  other  solutions  are  doubtless  possible. 

It  is  also  desirable  that  some  method  be  devised  for  obtaining,  at 
least  approximately,  the  radial  velocities  of  stars  possessing  spectra 
with  very  diffuse  lines.  At  the  present  time,  no  radial  velocities  have 
been  published  for  some  of  the  very  brightest  stars,  on  this  account. 


ASTRONOMY:  H.  N.  RUSSELL  IS 

In  extending  the  list  of  observed  radial  velocities,  much  advantage 
has  been  gained  by  a  policy  of  selective  observation  of  classes  of  stars  of 
special  interest — such  as  stars  of  unusually  large  and  small  proper  mo- 
tion, absolute  magnitude,  and  the  like,  variable  stars,  and  stars  of  the 
rarer  spectral  types.  A  similar  investigation  of  double  stars  showing 
evidence  of  physical  connection  would  be  worth  while. 

8.  Statistical  discussions  of  the  motions  of  the  stars  and  of  the  Sun,  and 
their  relation  to  spectral  type,  etc.,  offer  an  extensive  and  very  intricate 
field.    Among  the  matters  demanding  further  investigation  may  be 
mentioned  the  reason  for  the  differences  in  the  direction  and  velocity 
of  the  solar  motion  derived  from  stars  of  different  spectral  types,  and 
from  proper  motions  and  radial  velocities  separately;  the  origin  of  the 
constant  term  in  radial  velocities  (Campbell's  K  term) ;  the  existence  of 
tendencies  toward  common  motion  among  the  stars  in  particular  regions 
of  the  sky;  the  dependence  of  the  mean  peculiar  velocities  of  the  stars 
upon  spectral  type  and  absolute  magnitude,  and  the  real  cause  of  this 
dependence  (possibly  a  correlation  between  large  velocity  and  small 
mass);  the  true  nature  of  preferential  motion,  and  whether  it  really 
gives  evidence  of  the  existence  of  two  physically  different  'streams;' 
the  dependence  of  preferential  motion  upon  spectral  type,  absolute 
magnitude  (the  latter  an  unworked  field)  and  perhaps  upon  the  region 
of  the  sky  considered;  the  devising  of  a  rapid  method  for  the  detection 
of  moving  clusters,  and  the  identification  of  their  members;  and  so  on. 
The  discussion  of  most  of  these  problems  should  be  based  simultaneously 
on  proper  motions  and  radial  velocities.     Results  derived  from  either 
one  alone  may  fall  into  errors  which  the  combination  of  both  would 
detect. 

One  practical  matter  deserves  specific  mention.  When  it  appears 
desirable  to  exclude  certain  stars  from  a  statistical  discussion  (for  ex- 
ample, those  of  very  large  proper  motion),  the  limits  of  exclusion  should 
be  clearly  and  precisely  stated.  Neglect  to  do  so  may  cause  great 
trouble  to  other  workers  who  wish  to  make  a  comparison  with  their  own 
results,  and  has  sometimes  led  to  very  serious  errors  of  interpretation. 

9.  Another  set  of  data  of  fundamental  importance  depend  upon  rela- 
tions involving  the  masses  of  the  stars.    Here  there  appears  the  grave 
difficulty  that  nothing  at  all  can  at  present  be  found  out  concerning 
the  mass  of  a  star  unless  it  is  double.    There  are  plenty  of  double  stars, 
to  be  sure ;  but  what  certainty  have  we  that  they  are  similar  in  mass  to 
stars  which  are  not  double?    Only  an  indirect  answer  is  possible,  by 
means  of  the  statistical  comparison  of  single  and  double  stars  with  re- 
spect to  as  many  characteristics    as  may  be — absolute   magnitude, 


16  ASTRONOMY:  H.  N.  RUSSELL 

spectrum,  color,  radial  velocity,  proper  motion,  distribution  in  space, 
etc.  (bearing  in  mind  that  the  limits  of  telescopic  resolution  restrict 
our  knowledge  of  the  remoter  pairs).  But  Eddington's  recent  theo- 
retical researches82  lead  to  the  hope  that  it  may  some  day  be  possible  to 
estimate  the  mass  of  any  star  when  its  absolute  magnitude  and  spectral 
type  are  accurately  known  (using  the  data  for  double  stars  as  a  guide) . 

(a)  As  regards  the  determination  of  the  masses  of  individual  stars,  it 
should  be  borne  in  mind  that,  for  statistical  purposes,  a  pair  in  which  the 
relative  motion  of  the  components  is  known,  though  the  motion  in  angle 
may  be  only  a  few  degrees,  is  very  nearly  as  valuable  as  one  which  has 
completed  a  revolution — while  a  pair  for  which  the  relative  motion  is 
unknown  is  of  no  use  at  all.  The  slowly  moving  pairs  which  are  often, 
but  inaccurately,  described  as  'fixed/  possess  an  importance  exactly 
analogous  to  the  stars  of  small  proper  motion,  and  give  us  invaluable 
information  about  those  stars  which  are  bright  in  proportion  to  their 
mass — the  giant  stars,  in  fact.  Now  that  the  discovery  of  double  stars  is 
apparently  in  sight  of  completion,  it  is  to  be  hoped  that  more  attention 
may  be  given  to  the  problem  of  determining  the  relative  motion  in  as 
many  systems  as  possible. 

(6)  The  existing  data  suffice  to  show  that  the  masses  of  the  stars  differ 
from  one  another  less  than  any  other  of  their  characteristics — the  whole 
range  among  well  determined  masses  being  from  20  tunes  the  Sun's 
mass  to  one-sixth  of  the  Sun's,  which  may  be  compared  with  a  range  in 
luminosity  of  at  least  ten  million  fold.  For  this  very  reason,  very 
careful  observations  are  required  to  enable  us  to  say  with  certainty 
that  one  star  is  more  or  less  massive  than  another.  It  appears  certain 
that  the  stars  of  spectrum  B  are  unusually  massive,56  and  there  is  suffi- 
cient evidence  to  show  that,  hi  general,  stars  of  great  luminosity  are 
more  massive  than  those  of  small  absolute  brightness,  and  that,  among 
the  dwarf  stars,  those  of  'later'  spectral  type  are  of  smaller  average 
mass.67  But  there  are  very  few  cases  in  which  we  can  be  sure  that  a 
given  star  is  more  or  less  massive  than  the  average  for  its  type. 

It  is  very  desirable  to  determine  how  great  is  the  range  of  difference 
among  the  masses  of  stars  of  similar  spectral  class  or  absolute  magnitude. 
Extremely  precise  determinations  of  parallax  will  be  needed  if  this 
problem  is  to  be  solved,  but  the  effort  will  be  well  worth  while.  Suffi- 
ciently reliable  values  of  the  mean  masses  of  stars  of  different  groups 
have  already  been  determined,  to  make  it  possible  to  estimate  the 
parallaxes  of  all  but  the  nearer  binaries  and  'physical  pairs'  more 
accurately  than  they  can  at  present  be  observed.87  This  should  be  of 
aid  in  the  interpretation  of  other  statistical  studies  of  double  stars, 


ASTRONOMY:  H.  N.  RUSSELL  17 

such  as  the  proportion  of  double  stars  among  all  the  stars  of  a  given 
magnitude,  the  relative  numbers  of  close  and  wide  pairs,  etc. 

The  determination  of  the  relative  masses  of  the  components  of  binary 
systems  will  soon  also  be  possible  hi  many  cases  which  have  previously 
been  somewhat  neglected. 

When  a  sufficient  number  of  accurate  determinations  of  mass  have  been 
made,  a  detailed  study  of  the  spectra  of  stars  differing  in  mass  should 
be  made,  in  the  hope  of  finding  peculiarities  depending  directly  on  the 
mass,  which  might  make  it  possible  to  estimate  the  masses  of  isolated 
stars. 

(c)  A  great  number  of  spectroscopic  binaries  await  investigation,  and 
more  are  continually  being  discovered.  In  the  determination  of  orbits, 
preference  should  be  given  to  those  which  show  the  spectra  of  both 
components,  as  it  is  only  in  this  case  that  definite  information  can  be 
obtained  about  the  masses.  Eclipsing  and  Cepheid  variables  are  also 
worthy  of  special  attention,  and  also  stars  of  large  proper  motion,  or 
others  which  appear  to  be  dwarf  stars. 

It  is  very  desirable  that  some  method  should  be  found  for  observing 
the  spectrum  of  the  secondary  component  when  it  is  too  faint  to  be  di- 
rectly seen.  Perhaps  Koch's  spectromicrometer  might  furnish  a  solu- 
tion. Favorable  cases  for  trial,  in  which  the  brightness  of  the  invisible 
secondary  spectrum  is  known,  may  be  found  among  eclipsing  variables. 

10.  The  densities  of  stars  can  so  far  be  determined  only  when  they  are 
eclipsing  variables.    In  this  case,  when  both  spectra  can  be  photo- 
graphed, the  diameters  of  the  components  can  also  be  found.     Several 
systems  of  this  sort,  which  have  not  yet  been  investigated  spectro- 
graphically,  are  within  the  reach  of  existing  instruments. 

If,  however,  the  relations  between  spectral  type,  color  index,  and 
surface  brightness  can  be  so  well  determined  that  it  is  possible  to  esti- 
mate the  last  of  the  three  when  the  other  two  are  known,  it  will  then 
be  possible  to  determine  the  densities  of  all  visual  binary  stars,  the  linear 
diameters  of  all  stars  of  known  parallax,  and  the  angular  diameters  of 
all  the  stars  in  the  sky.  The  known  eclipsing  variables  should  afford 
sufficient  material  for  a  first  investigation  of  the  problem,  if  only 
sufficiently  accurate  information  can  be  obtained  regarding  the  color- 
equation  of  the  visual  and  photographic  methods  of  observation  which 
have  been  employed  at  various  observatories. 

11.  All  that  can  be  said  at  present  regarding  the  internal  constitution 
of  the  stars  depends  on  Eddington's  theoretical  work,82  which  indicates 
that,  in  the  stars  of  low  density,  the  mass  should  be  greatly  condensed 
toward  the  center — the  central  density  being  54  times  the  mean  den- 


18  ASTRONOMY:  H.  N.  RUSSELL 

sity.  But  the  problem  is  capable  of  investigation  by  observation. 
There  are  many  close  eclipsing  pairs  in  which  the  components  are  ellip- 
soidal in  form,  as  is  proved  by  variability  of  the  Beta  Lyrae  type.  In 
such  systems  the  lines  of  apsides  of  the  orbits  should  advance,  at  a 
rate  depending  on  the  masses,  dimensions,  and  internal  constitution 
of  the  components.  If  the  last  is  like  that  of  Jupiter  or  Saturn,  the 
advance  of  periastron  should  be  rapid.  What  little  evidence  there  is 
indicates  a  slower  motion,  and  hence  a  very  strong  central  condensation; 
but  more  intensive  studies  are  necessary  before  definite  conclusions 
can  be  drawn.  There  are  several  systems  for  which  the  necessary  data 
concerning  the  dimensions  and  forms  of  the  orbits  and  the  stars  are 
accessible  to  suitably  planned  observations, — notably  a  Virginis  and 
U  Herculis.  A  careful  study  of  such  stars,  by  means  of  simultane- 
ous photometric  and  spectroscopic  observations,  would  be  remunerative. 

The  singular  and  so  far  inexplicable  changes  which  occur  in  the 
periods  of  most  eclipsing  variables,  and  so  far  have  defied  prediction, 
also  deserve  extended  study;  and  Eddington  has  recently  called  atten- 
tion to  the  fact  that  secular  changes  in  the  periods  of  Cepheid  vari- 
ables are  likely  to  give  a  clue  to  the  rate  of  stellar  evolution.58  The 
first  scanty  evidence  points  to  a  very  extended  time  scale. 

12.  In  the  investigation  of  star-clusters,  measures  of  position,  for  the 
purpose  of  detecting  future  proper  motions,  are  obviously  a  duty  to  pos- 
terity. There  is  little  chance  that  anything  more  than  the  motion  of  the 
clusters  as  a  whole  will  be  perceptible  in  our  generation,  and  only  meas- 
ures of  the  utmost  attainable  accuracy  and  freedom  from  systematic  error 
are  likely  to  be  of  use  to  the  astronomers  of  the  future.  Of  far  more 
promise  are  studies  of  the  distribution  of  the  stars  within  the  clusters, 
their  magnitudes,  and,  above  all,  their  color  indices.  Such  investiga- 
tions, in  Shapley's  hands,59  have  given  us  for  the  first  time  a  true  con- 
ception of  the  distances  and  magnitudes  of  the  globular  clusters.  Stu- 
dents of  the  subject  are  eagerly  awaiting  the  detailed  publication  of  the 
evidence  on  which  he  bases  his  conclusion  that  the  apparent  avoidance 
by  these  clusters  of  the  region  within  1500  parsecs  of  the  galactic  plane 
is  due  to  a  real  absence  of  clusters  from  this  region,  and  not  to  obscura- 
tion by  absorbing  matter. 

The  variable  stars  in  clusters  also  deserve  further  attention.  Those  so 
far  discovered  appear  to  belong  to  the  Cepheid  type,  which  is  natural, 
as  these  seem  to  be  actually  the  brightest  of  all  variables.  Long  period 
and  eclipsing  variables  may  yet  be  discovered  among  the  fainter  stars. 

Good  work  can  still  be  done  also  upon  the  irregular  clusters, — as  is 
shown  by  Triimpler's60  study  of  the  outlying  members  of  the  Pleiades. 


ASTRONOMY:  B.  N.  RUSSELL  19 

One  of  the  most  attractive  of  unexplored  fields  is  the  investigation  of 
the  Magellanic  Clouds.  The  small  amount  of  work  which  has  been  done, 
mainly  on  the  Smaller  Cloud,  has  led  to  the  discovery  of  a  remarkable 
relation  between  the  periods  and  absolute  magnitudes  of  the  variables 
in  the  Cloud,61  to  the  estimate  that  its  distance  is  20,000  parsecs,62  and 
to  the  discovery  that  the  nebulae  within  it,  and  probably  the  Cloud  as  a 
whole,  have  a  very  high  radial  velocity.63  The  great  instruments  which 
are  now  being  erected  in  the  southern  hemisphere  may  well  be  actively 
directed  toward  this  region. 

13.  (a)  Foremost  among  the  many  problems  presented  by  the  gaseous 
nebulae  is  the  cause  of  their  luminosity.  In  spite  of  our  ignorance  of  the 
origin  of  the  characteristic  nebular  lines,  the  appearance  of  such  lines  as 
X  4686  in  the  spectra  of  nebulae,  and  of  the  Wolf-Rayet  spectrum  in  their 
nuclei,  suggests  that  in  them  "we  are  presented"  (in  Fowler's  words)64 
"with  phenomena  which  result  either  from  the  effects  of  powerful  elec- 
trical actions  or  of  very  elevated  temperatures."  Though  such  condi- 
tions may  easily  enough  exist  in  the  nuclei,  it  is  very  hard  to  see  how 
high  temperatures  can  prevail  throughout  the  whole  volume  of  a 
nebula.*  There  are  several  possible  ways  out,  however. 

The  electrical  action  may  be  a  bombardment  of  the  outer  region  by  cor- 
puscles emitted  from  the  nucleus.  Or  perhaps  the  luminosity  of  the  gases 
is  fluorescent,  like  that  of  the  sodium  or  bromine  vapors  studied  by 
Wood.68  Or,  as  Fabry  has  recently  suggested,67  we  may  have  to  do  with 
a  body  which  absorbs  and  emits  radiation  only  in  narrow  regions  of 
short  wave-length,  and  may  therefore  attain  a  very  high  temperature  hi 
thermal  equilibrium  with  the  radiations  from  a  distant,  but  still  hotter, 
source.  To  determine  the  true  explanation  among  these  and  many 
other  possibilities  may  tax  the  resources  of  both  experimental  and 
theoretical  spectroscopy. 

The  association  of  gaseous  nebulae  with  stars  of  'early'  spectral  type 
might  be  anticipated  on  any  of  these  theories.  For  such  stars  are  very 
hot  bodies,  and  would  be  the  most  powerful  sources  both  of  corpuscular 
and  ultra-violet  radiation.  Hence  the  association  of  these  stars  with 
nebulae  does  not  prove  that  the  stars  originate  from  the  nebulae.  It  is 
entirely  conceivable  that,  on  the  contrary,  the  nebulae,  as  visible  ob- 
jects, owe  their  existence  to  the  radiation  of  the  stars,  and  are  their 
offspring,  and  not  their  parents.  Some  gaseous  nebulae,  however,  are 
not  near  bright  stars,  and  the  nuclei  of  planetary  nebulae  appear  to  be 

*  Fabry's  calculated  temperature  of  15,000°  for  the  Orion  Nebula,*5  as  he  points  out, 
is  liable  to  be  diminished  by  an  unknown  amount  on  account  of  the  widening  of  the  lines 
of  the  spectrum  by  turbulent  motion  of  the  nebular  matter  in  the  line  of  sight. 


20  ASTRONOMY:  H.  N.  RUSSELL 

comparable  with  some  of  the  faintest  stars  in  luminosity.  Clearly,  noth- 
ing final  can  be  said  on  this  subject  until  we  know  what  it  is  that  shines 
in  the  gaseous  nebulae,  and  why.  It  may  be  remarked,  however,  that 
the  wide-spread  assumption  that  the  origin  of  the  stars  is  to  be  sought 
in  the  visible  nebulae  appears  to  have  had  very  little  solid  basis.  All 
classes  of  nebulae  except  the  extended  gaseous  nebulae  have  already 
been  excluded  from  consideration  as  observational  knowledge  increased. 

(&)  A  few  nebulae,  like  those  in  the  Pleiades,68  appear  to  shine  by  light 
reflected  from  neighboring  stars,  and  Slipher's  spectroscopic  work  is 
steadily  adding  to  the  list  of  his  discoveries  in  this  field.  Hertzsprung69 
has  shown  photometrically  that  the  brightness  of  the  nebulosity  in  the 
Pleiades  is  entirely  consistent  with  the  reflection  hypothesis.  Similar 
studies  of  other  nebulae,  and  especially  of  the  remarkable  variable 
nebulae  recently  observed  by  Slipher,70  would  be  of  value. 

Barnard's  long  continued  researches71  have  made  it  highly  probable 
that  there  exist  many  dark  nebulae,  revealed  only  by  the  effects  of  their 
opacity  in  concealing  whatever  lies  beyond  them.  It  is  highly  signifi- 
cant that  the  most  remarkable  of  these  dark  regions  is  obviously  directly 
connected  with  one  of  the  nebulae  which  shines  by  reflected  light, — 
that  surrounding  Rho  Ophiuchi72 — and  that  the  whole  mass  is  com- 
paratively near  us  in  space,  at  a  distance  of  100  to  150  parsecs.  If 
such  masses  of  practically  opaque  material  are  scattered  through  the 
galactic  and  extra-galactic  regions  at  distances  comparable  with  this, 
the  resulting  absorption  of  light  must  play  a  very  important  r61e  in 
limiting  the  apparent  extent  of  the  universe.  Jf  this  absorption  is  of  the 
type  which  is  produced  by  dust,  or  even  by  particles  of  the  size  of  the 
drops  of  water  in  ordinary  clouds,  it  will  affect  all  wave-lengths  to 
substantially  the  same  extent,  and  be  much  more  difficult  to  detect 
than  the  gaseous  scattering,  increasing  for  the  shorter  wave-lengths, 
which  several  investigators  have  sought  for,  but  whose  existence  Shapley 
has  apparently  disproved.22  It  seems  appropriate  to  remark  in  this 
connection  that  absorption  independent  of  the  wave  length  seems 
a  priori  much  more  likely  to  occur  than  the  other,  since  the  same  quan- 
tity of  matter  in  the  form  of  a  fog  is  incomparably  more  effective  than 
in  gaseous  form,  (compare  the  opacity  of  a  few  meters  of  cloud  with 
that  of  all  the  rest  of  the  atmosphere)  and  also  since  most  forms  of 
matter  are  likely  to  be  in  the  solid  or  liquid  state  at  the  temperatures 
prevailing  in  interstellar  space. 

(c)  The  forms  of  nebulae — especially  of  planetary  and  ring  nebulae- 
deserve  careful  study.  As  Campbell  suggests,73  it  is  difficult  to  account 
for  them  without  assuming  the  existence  of  some  repulsive  force  which 


ASTRONOMY:  H.  N.  RUSSELL  21 

counteracts  the  attraction  of  the  nucleus.  He  suggests  light-pressure — 
which  would  fit  in  well  with  views  of  the  origin  of  the  luminosity  such 
as  are  suggested  above.  In  such  a  case  we  should  anticipate  that  most 
of  the  light  of  the  nebula  would  come  from  the  nucleus,  and  this  appears 
to  be  usually,  though  not  always,  the  case. 

(d)  Measures  of  the  radial  velocities  of  nebulae  have  already  shown  that 
the  planetary  nebulae,  as  a  class,  are  moving  in  space  much  more  rapidly 
than  the  stars;7*  that  there  exist  internal  motions  within  them,  usually 
of  a  rotational  character,  but  sometimes  more  complicated  ;73  and  that, 
in  order  to  keep  the  moving  material  from  flying  away  into  space,  the 
total  masses  of  the  nebulae  must  be  very  considerable,  and  probably  a 
good  deal  larger  than  those  of  the  stars.73  Much  remains  to  be  done 
in  the  investigation  of  these  motions,  and  in  their  interpretation.  The 
proper  motions  of  planetary  nebulae,  and  perhaps  in  some  cases  the 
internal  motions  of  the  nebular  material,  can  be  determined  by  compari- 
son of  suitable  photographs,  and  it  is  probable  that  in  a  decade  or  two 
we  shall  obtain  in  this  way  a  fair  idea  of  the  distances  and  real  dimensions 
of  these  bodies.  Observations  for  parallax  on  some  of  the  larger  and 
presumably  nearer  planetary  nebulae  are  also  desirable. 

The  extended  gaseous  nebulae  should  be  examined  spectrographically 
to  see  whether  turbulent  motions  exist  in  others,  as  they  do  in  the  great 
nebulae  of  Orion;76  and  it  would  be  worth  while  to  compare  photographs 
of  some  of  those  which  show  sharp  details,  in  the  hope  of  detecting  proper 
motion,  either  of  the  whole  or  of  parts. 

Investigations  of  the  distribution  within  the  gaseous  nebulae  of  the 
substances  which  give  the  different  spectral  lines  may  be  made  by 
photography  either  with  absorbing  screens  or  with  slitless  spectro- 
scopes, and  promise  information  regarding  the  conditions  prevailing  in 
the  nebulae,  and  the  mutual  relations  of  the  lines  of  unknown  origin. 

14.  (a)  The  spiral  nebulae  have  been  shown  by  recent  investigations  to 
be  the  most  extraordinary  objects  in  the  heavens.  Their  enormous  radial 
velocities — first  detected  by  Slipher76 — and  the  almost  equally  rapid 
internal  motions  within  them,77  put  them  in  a  class  by  themselves. 
Further  measures  of  these  motions  are  needed;  and,  when  the  radial 
velocities  of  a  sufficient  number  of  spirals,  well  distributed  over  the 
heavens,  are  known,  it  may  be  possible  to  determine  definitely  the 
direction  and  rate  of  the  motion  of  the  Sun  (and  presumably  of  the 
whole  galactic  system)  with  respect  to  the  system  of  nebulae.  The 
provisional  determination  by  Young  and  Harper,78  from  very  scanty 
data,  indicates  for  the  motion  of  our  system  the  enormous  velocity  of 
600  kilometers  per  second. 


22  ASTRONOMY:  B.  N.  RUSSELL 

(b)  As  van  Maanen79  and  others80  have  shown,  the  proper  motions  of 
some  spiral  nebulae — both  of  the  mass  as  a  whole  and  of  the  condensa- 
tions in  the  arms  relatively  to  the  centre — are  apparently  large  enough 
to  be  determined  by  the  careful  comparison  of  plates  taken  only  a  few 
years  apart.    This  opens  up  another  wide  field  of  study,  and  will  make 
it  possible  before  long  to  determine  the  mean  parallax  of  many  such 
nebulae  by  comparison  of  the  proper  motions  and  radial  velocities  of 
their  nuclei.    There  is  also  reason  to  hope  that  the  distances  of  some 
individual  nebulae,  which  are  seen  at  a  suitable  angle,  can  be  deter- 
mined by  comparing  the  radial  and  transverse  components  of  motion 
along  the  arms.    Enough  is  already  known  to  convince  us  that  the 
distances  of  these  nebulae  must  be  measured  in  thousands  of  parsecs, 
and  their  diameters  in  parsecs,  and  that  direct  measures  for  parallax 
are  utterly  hopeless. 

(c)  Photometric  measures,  both  of  the  total  light  of  the  spirals  and  the 
relative  brightness  of  their  parts,  would  be  of  value,  especially  if  accom- 
panied by  determinations  of  color.     Scares81  has  recently  shown  that  the 
outer  convolutions  are  far  bluer  than  the  centre — which  is  the  part  that 
shows  the  spectrum  of  solar  type.     Spectroscopic  observations  of  these 
outer  regions,  if  possible,  would  be  of  great  interest.    Another  matter 
calling  for  further  study  is  the  nature  of  the  dark  bands  which  cross 
many  nebulae  which  appear  to  be  spirals  seen  edgewise,  and  look  as  if 
they  were  due  to  the  interposition  of  opaque  material  in  the  outer 
regions  of  the  nebula. 

( d)  The  distribution  of  spiral  nebulae  in  the  heavens — so  utterly  differ- 
ent from  that  of  any  other  objects — may  be  explainable  when  their  real 
distribution  in  space  is  even  partially  known.     It  is  hardly  time  as 
yet  to  consider  the  greater  question  of  their  real  nature,  except  to  note, 
with  van  Maanen,79    that,  unless  they  are  in  process  of   very  rapid 
dissipation  into  space,  their  masses  must  be  exceedingly  great. 

15.  Finally,  it  must  not  be  forgotten  how  important  a  place  theoretical 
investigations  will  occupy  in  the  solution  of  the  larger  problems  of  sidereal 
astronomy.  The  increasing  observational  data  are  already  furnishing 
just  those  guides  which  point  the  skilled  mathematician  in  the  right 
direction,  and  these  indications  have  been  very  successfully  followed, 
especially  by  certain  members  of  that  'Cambridge  school'  which  com- 
bines keen  mathematical  analysis  with  a  thorough  knowledge  of  modern 
physics.  Results  of  remarkable  generality  have  already  been  obtained. 

In  the  field  of  stellar  evolution,  Eddington82  has  worked  out  in  de- 
tail the  importance  of  radiation  pressure  in  determining  the  conditions 
of  internal  equilibrium  of  the  stars,  and  the  approximate  equality  in 


ASTRONOMY:  H.  If.  RUSSELL  23 

brightness  of  the  giant  stars  of  all  spectral  types  has  found  a  simple 
explanation. 

If  the  conclusion  that  the  luminosity  of  a  giant  star  is  a  function  of  its 
mass,  but  not  of  its  temperature  or  age,  is  confirmed,  and  the  nature  of 
the  function  fixed  by  observation,  the  problem  of  determining  the  masses 
of  stars  which  are  not  double  will  in  many  cases  be  solved. 

Jeans,83  discussing  the  problem  of  the  figures  of  equilibrium  of  a  rotat- 
ing mass  of  compressible  fluid,  has  already  reached  conclusions  which  not 
only  bear  upon  the  origin  of  double  stars,  but  have  suggested  an  entirely 
new  and  very  stimulating  conception  of  the  nature  of  spiral  nebulae,  as 
huge  rotating  masses  of  gas,  which,  becoming  unstable  at  the  edge  under 
the  influence  of  their  own  rotation  and  the  attraction  of  the  neighboring 
stars,  throw  off  matter  from  their  periphery  in  streams  of  such  enormous 
size  that  they  may  divide  into  'nuclei'  large  enough  to  form  ordinary 
stars  upon  condensation. 

In  the  field  of  galactic  astronomy,  Schwarzschild84  has  developed 
powerful  methods  for  handling  the  statistical  material  which  must  be 
our  main  guide,  and  Jeans86  and  Eddington86  have  shown  that  'star 
streaming'  demands  no  unknown  forces  for  its  explanation,  but  is  prob- 
ably interpretable  dynamically,  as  a  property  of  a  system  of  stars  in 
motion  under  their  own  gravitation — although  the  existence  of  'stream- 
ing' appears  to  indicate  that  the  galactic  system  is  not  in  a '  steady  state.' 
Eddington87  has  shown  that  the  similarity  of  distribution  of  the  stars  in 
different  globular  clusters  presents  a  problem  by  no  means  simple,  though 
of  much  interest. 

Almost  the  whole  of  this  work  has  appeared  within  the  last  three  years, 
and  further  notable  advances  may  be  anticipated.  Indeed,  almost  as 
these  words  are  written,  comes  the  first  installment  of  an  important 
paper  by  Eddington88  on  the  oscillations  of  a  gaseous  star,  which  may 
afford  the  long-sought  solution  of  the  problem  of  Cepheid  variation. 

Among  other  specific  problems  awaiting  discussion  may  be  mentioned 
the  question  whether  the  tidal  interaction  of  two  compressible  and 
slowly  condensing  bodies  can  cause  an  originally  small  eccentricity  to 
increase  to  the  very  large  values  which  are  found  in  many  visual  binaries, 
and  some  spectroscopic  binaries  as  well;  and,  if  this  proves  to  be  impos- 
sible, how  the  systems  in  question  can  have  originated;89  the  origin  and 
laws  of  the  complicated  changes  which  occur  in  the  periods  of  many 
eclipsing  binaries;  and  the  equilibrium  and  motions  of  the  constituent 
parts  of  planetary  and  spiral  nebulae. 

Mention  should  also  be  made  of  the  work  of  Nicholson"  on  the 
interpretation  of  unknown  lines  in  the  spectra  of  nebulae  and  of  the 


24  ASTRONOMY:  H.  N.  RUSSELL 

solar  corona  as  arising  from  hypothetical  atoms  of  very  simple  structure 
— which  has  successfully  met  the  test  of  prediction — and  of  the  develop- 
ment of  the  theory  of  general  relativity,  which  has  already  been  used  by 
deSitter94  to  set  a  superior  limit  to  the  whole  quantity  of  matter  in  the 
universe,  and  may  have  important  applications  in  future. 

16.  Of  more  fundamental  nature,  and  obvious  importance,  is  the  un- 
solved problem  of  the  source  of  the  energy  which  the  stars  are  continually 
radiating  at  so  rapid  a  rate.  It  is  becoming  increasingly  plain  that  the 
gravitational  energy  liberated  by  contraction  from  infinity  would  not 
nearly  suffice  to  maintain  the  Sun's  radiation  during  geological  time90 
(according  to  even  the  more  conservative  estimates  of  the  latter);  yet 
the  mere  continuous  existence  of  life  on  the  Earth  is  evidence  that  the 
Sun  has  not  merely  kept  on  shining  throughout  this  interval,  but  has 
not  changed  in  brightness  by  more  than  one  magnitude,  at  the  outside. 
In  the  case  of  some  giant  stars,  contraction  from  infinity  would  hardly 
suffice  to  furnish  the  energy  which  they  have  radiated  during  historic 
time.91  There  appear  to  be  two  ways  out  of  the  difficulty;  either  the 
stars  do  not  radiate  heat  in  all  directions  to  space  at  the  same  rate  as 
they  do  towards  the  Earth,  or  else  they  have  some  unknown  and  ex- 
ceedingly great  supplies  of  internal  energy.  The  first  alternative, 
however,  seems  to  be  excluded  by  the  fact  that  the  amount  of  heat 
which  the  Earth  receives  from  the  Sun,  and  loses  again  by  radiation 
into  space,  is  not  greatly,  and  probably  not  at  all,  inferior  to  that 
which  a  black  body  of  the  same  size  and  temperature  as  the  Earth's 
effective  radiating  surface  would  radiate  to  an  enclosure  at  the  absolute 
zero.92  There  seems  therefore  no  escape  from  the  conclusion  that  the 
heat  radiated  by  a  star  can  not  be  provided  by  contraction.  What  the 
source  of  the  energy  may  be,  how  it  is  converted  into  heat  in  the  body 
of  the  star,  and  where  it  goes  after  passing  from  the  star's  surface  into 
the  ether,  are  at  present  the  greatest  of  all  the  unsolved  problems  of 
astronomy. 

1  Xewall,  Baxandall  and  Butler,  Monthly  Not.  Roy.  A  sir  on.  Soc.,  London  76,  1916,  (640). 
1  Fowler,  A.,  Proc.  Royal  Soc.,  London,  A,  94,  1918,  (470). 

•  Fowler,  A.,  Ibid.,  A,  94,  1918,  (472). 

4  Fowler  and  Brooksbank,  Monthly  Not.  R.  A.  S.t  London,  77,  1917,  (511-517). 

•  Astropky.  /.,  Chicago,  40,  1914,  (466-472). 

•  Curtiss,  R.  H.,  Popular  Astronomy,  Northfield,  Minn.,  25,  1917,  (279-285). 
7  Cannon,  Miss  A.  J.,  Ibid.,  24,  1916,  (656). 

I  Adams,  W.  S.,  these  PROCEEDINGS,  2,  1916,  (143). 

9  Hertzsprung,  E.,  Z.  Wiss.  Photog.,  Leipzig,  3,  1905,  (429-435). 

10  Adams,  W.  S.,  these  PROCEEDINGS,  2,  1916,  (147-156). 

II  See  Ref.  6;  also  Rufus,  W.  C.,  Pub.  Astron.  Obs.  Univ.  Michigan,  Ann  Arbor,  2,  1916, 
(143). 


ASTRONOMY:  H.  N.  RUSSELL  .  25 

B  King,  E.  S.,  Ann.  Harvard  Coll.  Obs.,  Cambridge,  76,  1915,  (119). 

"Merrill,  P.  W.,  Popular  Astron.,  Northfield,  25,  1917,  (661). 

14  Adams,  W.  S.,  Astroph.  J.,  Chicago,  33,  1911,  (64-71). 

18  Coblentz,  W.  W.,  Lick.  Obs.  Bulletin,  Berkeley,  Col.,  8,  1915,  (104-120). 

16  Wilsing  and  Scheiner,  Publ.  Astrophys.  Obs.,  Polsdam,  19,  1909,  (5-221). 

17  Rosenberg,  H.,  Astron.  Nach.  Kiel,  193,  1912,  (357-370). 

18  See  Harvard  Annals,  64,  126-127. 

19  King,  E.  S.,  Ibid.,  76,  1915,  (189). 

20  Hertzsprung,  E.,  Astrophys.  Jour.,  42,  1915,  (92-110). 

21  Scares,  F.  H.,  these  PROCEEDINGS,  2,  1916,  (521). 

22  Shapley,  H.,  these  PROCEEDINGS,  2,  1916-17,  (12-15)  and  3,  (267-270). 

23  King,  L.  V.,  Trans.  Roy.  Soc.  Canada,  Ottawa,  9,  1915,  (99-103). 

24  Color  index  of  f  Persei  +  0.16;  color  index  of  £  Persei  +  0.03;  King,  E.  S.,  Harvard 
Annals,  76,  1915,  (118).   Color  index  of  o  Persei;  similar  to  that  of  a  star  of  spectrum  F., 
Wilsing  and  Scheiner,  Potsdam  Publ.  19,  1909,  (64). 

25  Barnard,  E.  E.,  Astrophys.  Jour.,  41,  1915,  (253-258). 
*  Coblentz,  W.  W.,  Lick  Obs.  Bull.,  8,  1915,  (121). 

27  Hertzsprung,  E.,  Astrophys.  Jour.,  42,  1915,  (352-356). 

28  Harvard  Circular,  No.  188,  1915. 

29  Cannon,  Miss  A.  J.,  Harvard  Annals,  91,  1918,  (274),  (star  No.  1546). 

30  Stebbins,  J.,  Pop.  Astron.,  25,  1917,  (657),  and  earlier  papers. 

31  Guthnick,  P.,  V ' ierteljahrsschrift  d.  Astron.  Gesellsch.,  53,  1918,  (169). 

32  Shapley,  H.,  Astrophys.  Jour.,  43,  1916,  (180). 

33  Cannon,  Miss  A.  J.,  Pop.  Astron.,  25,  1917,  (314). 

34  Shapley,  H.,  Astrophys.  Jour.,  44,  1916,  (273-291). 
»  Shapley,  H.,  Ibid.,  40,  1914,  (448-465). 

36  See  numerous  notes  by  Mrs.  W.  P.  Fleming,  Harvard  Annals.  56,  (209-212). 

37  Adams,  W.  S.  and  Pease,  F.  G.,  these  PROCEEDINGS,  1,  1915,  (391). 

38  Cannon,  Miss  A.  J.,  Harvard  Annals,  76,  57. 

39  Hertzsprung,  E.,  Astrophys.  Jour.,  42,  1915,  (118). 

40  Kapteyn,  J.  C.,  Ibid.,  40,  1914,  (118-126). 

41  Russell,  H.  N.,  and  Shapley,  H.,  Ibid.,  40,*  1914,  (422-423). 

42  Shapley,  H.,  Ibid.,  48,  1918,  (282-287). 

43  Hertzsprung,  E.,  Zeitschr.f.  Wiss.  Phot.,  3,  1905,  (442). 

44  Russell,  H.  N.,  Pop.  Astron.,  22,  1914,  (275-294  and  331-357). 

45  Adams,  W.  S.,  these  PROCEEDINGS,  2,  1916,  (157-163). 
"Kapteyn,  J.  C.,  Astrophys.  Jour.,  47,  1918,  (255-275). 

47  Russell,  H.  N.,  Observatory,  London,  37,  1914,  (167-169). 

48  Stromberg,  G.,  Astrophys.  Jour.,  47,  1918,  (14). 

49  For  example,  the  mean  absolute  magnitude  given  by  Adams,  (Mt.  Wilson  Contribution, 
No.  142)  for  the  four  Cepheid  variables  and  eight  other  stars  of  Miss  Maury's  classes  c 
and  ac  which,  appear  in  his  list,  is  +  0.3;  and  there  is  abundant  evidence  from  the  parallactic 
motions  that  the  mean  absolute  magnitude  of  the  stars  of  this  peculiar  class  is  at  least  as 
bright  as  —  2,  and  probably  brighter. 

60  Kapteyn,  J.  C.,  Astrophys.  Jour.,  47,  1918,  (263). 

61  Adams,  W.  S.,  Publ.  Astron.  Soc.  of  the  Pacific,  27,  1915,  (236). 

62  Adams,  W.  S.,  Ibid.,  26,  1914,  (198). 

63  Comstock,  G.  C.,  Astron.  Journal,  Albany,  25,  1907,  (119). 

64  Barnard,  E.  E.,  Ibid.,  29,  1916,  (181). 

58  See  summary  in  Monthly  Not.  Roy.  Ast.  Soc.,  78,  1918,  (304). 
»  Ludendorff ,  H.,  Astron.  Nach.,  189,  1911,  (145-155). 

67  Russell,  H.  N.,  Pop.  Astron.,  25,  1917,  (666). 

68  Eddington,  A.  S.,  MonthlyMot.  Roy.  Ast.  Soc.,  79,  1918,  (19). 


26  ASTRONOMY:  ff.  N.  RUSSELL 

89  Shapley,  H.,  Astrophys.  Jour.,  48,  1917-18,  (89-124,  154-181). 
60Trumpler,  R.,  Pop.  Astron.,  26,  1918,  (9). 

61  Leavitt,  Miss  H.,  Harvard  Circular,  173,  1912. 

62  Shapley,  H.,  Astrophys.  Jour.,  48,  1918,  (155). 

63  Wilson,  R.  E.,  Pub.  Ast.  Soc.  Pacific,  27,  1915,  (86). 

64  Fowler,  A.,  Monthly  Not.  Roy.  Ast.  Soc.,  77,  1917,  (516). 
'  K  Fabry,  Astrophys.  Jour.,  40,  1914,  (256). 

66  Wood,  R.  W.,  Physical  Optics,  2d  ed.  (New  York,  1911),  p.  583. 

87  Fabry,  Astrophys.  Jour.,  45,  1917,  (269-277). 

93  Slipher,  V.  M,,  Lowell  Observatory  Bulletin,  2,  1912,  (24). 

69  Hertzsprung,  E.,  Ast.  Nach.,195,  1913,  (449). 

70  Slipher,  V.  M.,  Lowell  Obs.  Bull.,  3,  1918,  (63). 

71  Bannard,  E.  E.,  Astrophys.  Jour.,  49,  1919,  (1). 

72  Slipher,  V.  M.,  Lowell  Obs.  Bull.,  2,  (155). 

73  Campbell,  W.  W.,  Pub.  Ast.  Soc.  Pacific.,  29,  1916,  (284). 

74  Campbell,  W.  W.  and  Moore,  J.  H.,  Ibid.,  27,  1915,  (240). 

75  Fabry,  Astrophys.  Jour.,  40,  1914,  (241).  «    . 

76  Slipher,  V.  M.,  Pop.  Astron.,  23,  1915,  (23). 

77  Slipher,  V.  M.,  Lowell  Obs.  Bull.,  2,  1914,  (66). 

78  Young  and  Harper,  Jour.  Roy.  Ast.  Soc.  Canada,  10,  (134.) 
70  van  Maanen,  A.,  Astrophys.  Jour.,  44,  1916,  (210-228). 

80  Kostinsky,  S.,  Monthly  Not.  Roy.  Ast.  Soc.,  77,  1917,  (233). 

81  Scares,  F.  H.,  these  PROCEEDINGS,  2,  1916,  (553). 

82  Eddington,  A.  S.,  Monthly  Not.  Roy.  Ast.  Soc.,  77,  1916-17,  (16  and  596);  also  As- 
trophys.  Jour.,  48,  1918,  (205). 

83  Jeans,  J.  H.,  Observatory,  40,  1917,  (60). 

84  Schwarzschild,  K.,  Aslron.  Nach.,  190,  1912,  (361),  and  198,  1914,  (217). 

85  Jeans,  J.  H.,  Monthly  Not.  Roy.  Ast.  Soc.,  76,  1915-16,  (70  and  552). 

86  Eddington,  A.  S.,  Ibid.,  74,  1913,  (5),  and  75,  1916,  (366),  and  76,  1916,  (37). 

87  Eddington,  A.  S.,  Ibid.,  76,  1916,  (572-585). 

88  Eddington,  A.  S.,  Ibid.,  79,  1918,  (2). 

89Nolke,  F.,  Abh.  Nat.  Ver.  Bremen,  20,  Teil  2,  1911,  (193-220).  Jeans,  J.  H.,  Obser- 
vatory, 42,  1919,  (40). 

90  Shapley,  H.,  Pub.  Ast.  Soc.  Pacific.,  October,  1918,  (283-298). 

91  Eddington,  A.  S.,  Monthly  Not.  Roy.  Ast.  Soc.,  79,  1918,  (19). 

92  Abbot,  G.  G.,  these  PROCEEDINGS,  4,  1918,  (104-106). 

93  Nicholson,  J.  W:,  Monthly  Not.  Roy.  Asl.  Soc.,  76, 1916,  (415);  78, 1918,  (349),  and  earlier 
papers. 

94  deSitter,  W.,  Ibid.,  78,  1917,  (25). 

96  As  this  paper  goes  to  press,  van  Maanen,  (Pub.  Ast.  Soc.  Pacific,  31,  1919,  (42)  )  an- 
nounces the  discovery  of  a  star  of  type  F,  and  absolute  magnitude  14.3  on  Kapteyn's  scale. 
If  the  surface  brightness  of  this  object  has  the  value  which  is  ordinarily  associated  with  its 
color  and  spectrum,  its  diameter  must  be  comparable  with  that  of  the  Earth.  These  faint 
stars  are  evidently  going  to  present  a  problem  of  great  interest  and  difficulty. 


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